1. Field of the Invention
The present invention relates to active power line conditioners which are utilized to regulate the quality of electrical energy delivered from an electrical energy source to an electrical load. More particularly, the invention relates to a series-parallel active power line conditioner (APLC) which utilizes a reduced-turns-ratio transformer to achieve significant improvements in peak voltage regulation capability.
2. Description of the Prior Art
Electric supply networks are increasingly being exposed to the consequences of nonlinear loads, such as data processing equipment, numerical controlled machines, variable speed motor drives, robotics, medical apparatus and communication equipment. Such loads draw nonlinear pulse-like currents instead of the sinusoidal currents drawn by linear loads (i.e., resistors, inductors and capacitors). These nonlinear currents flow through the source impedance, thus causing distortion of the AC line voltage.
This voltage distortion may produce a number of undesired effects. For example, sensitive loads connected to the network may experience operational difficulties. Additionally, the RMS current supplied by the source increases due to the presence of harmonics in the pulse-like currents. These harmonic currents may significantly increase I.sup.2 R losses in interposing transformers.
Another problem which may have significant effects on many types of electrical equipment is the occurrence of temporary sags in the AC line voltage. For example, many types of electrical equipment utilize a power supply input stage which converts the AC line voltage to DC voltage utilizing a full wave rectifier connected across one or more large filter capacitors. In normal operation, the filter capacitor recharges with each peak of the rectified line voltage. It is only during this peak that the load is actually drawing current from the electrical supply network. When the rectified line voltage is lower than the voltage level on the filter capacitors, the rectifier diodes will prevent current from flowing. If, however, the AC line voltage does not maintain an adequate peak-to-peak value, these filter capacitors will not be able to maintain their required peak charge levels. As a result, operation of equipment incorporating the rectified capacitive power supply may be affected.
The effects of many of these problems can be mitigated through the use of power electronic devices known as active power line conditioners. Such devices typically comprise one or two switching inverters arranged in a series, parallel, or series-parallel configuration. The inverters are controlled (generally by pulse width modulation (PWM) techniques) to effect a flow of current between a DC energy storage element and the AC supply lines to which they are connected. Such devices are shown and described in U.S. Pat. Nos. 4,651,265 and 3,825,815, which are incorporated herein by reference.
When a single inverter is used, this current may consist of the harmonic and ripple currents required by the load. In a series-parallel configuration, two inverters are arranged to share a common DC link. In this arrangement, the inverters may cooperate to effect a transfer of real power between the source or load and the DC link. This may be helpful to insure that the load continually sees ideal current and voltage characterisrics.
The series inverter in a series-parallel APLC is typically coupled to the AC supply line via a coupling transformer. The series inverter applies a voltage of selected magnitude and polarity to the secondary winding of the transformer, which produces an AC regulation voltage on the primary winding of the transformer. This AC regulation voltage bucks or boosts (i.e., is added to or is subtracted from) the AC supply voltage to maintain the AC output voltage seen by the load at a regulated nominal value. The parallel inverter of the series-parallel APLC may operate as an active filter to supply real and reactive currents to the load as needed as well as cooperating with the series inverter to effect real power transfer.
The voltage regulation capability of a series-parallel active power line conditioner is given in terms of a rated buck/boost voltage range. While the line voltage magnitude stays within the rated buck/boost range, the output voltage stays generally constant and sinusoidal at the nominal value. Typically, the rated buck/boost range of an active power line conditioner is expressed as a percentage of the nominal output voltage. Generally, such range is selected to fall between .+-.10% to .+-.25% of the nominal output voltage. (The voltage buck/boost range tends to be symmetrical due to symmetry of the series inverter power circuit.)
When the supply voltage sinks below the rated boost range of the APLC, however, the output voltage tends to also be dragged down. Thus, the voltage seen by the various loads connected to the electrical supply network will fall below the nominal value. As a result, the quality of power delivered to these loads is no longer insured. This is particularly true in the case of loads having rectified capacitive inputs, since the filter capacitors may not be able to obtain their peak charge level. Additionally, transient voltage sags or surges may exceed the selected rating of the APLC on a statistical basis.
In order, therefore, to provide the greatest assurance of power quality to loads supported by the series-parallel APLC, it is desirable for the device to have as large buck/boost regulation range as is practical. The weight and cost, however, of many of the APLC components are proportional to the regulation range. As such, an APLC with a higher regulation range would be expected to be larger and more expensive than a similar device with a smaller regulation range. This may tend to negatively affect the commercial viability of such equipment. The installation of a larger coupling transformer to support excessive input voltage sags would also have double iron losses that would adversely affect overall efficiency.